The present invention relates to a glass composition possessing remarkable alkali-resistance. In general, silicate glass possesses poor alkali-resistance and thus has a defect of being attacked by an alkaline solution. Especially, in case glass fibers with a large specific surface area are used to reinforce such alkaline cementitious products as cement or hydrated calcium silicate, alkali-resistance of the glass fibers is taken up as a very important problem for durability of the fiber-reinforced composite products.
Hitherto, natural asbestos has widely been used as reinforcing fibers for alkaline cementitious products. At present, however, the use of natural asbestos is very strictly limited because its resource is becoming exhausted and its carcinogenicity is strongly suspected in recent years. Under such circumstances, there is a great demand for development of inorganic, non-combustible and alkali-resistant glass fibers capable of being substituted for natural asbestos.
In view of such a severe situation, worldwide extensive research and development of alkali-resistant glass fibers are now being carried out actively and some studies thereon have been reported already. However, glass compositions proposed according to the prior studies have one or more drawbacks including poor alkali-resistance, difficulty in manufacture due to high temperatures required for fusion or fiberization, devitrification and high cost. Up to the present, therefore, none of these prior art glass compositions has been used for the manufacture of glass fibers on a truly commercial basis. Especially from the economical point of view, it is desirable that the starting materials should be supplied in a large amount at a low cost.
As a result of various researches made to solve the various problems referred to above, it has now been found that a specific range of composition capable of affording an extremely high alkali-resistant glass can be obtained by incorporating natural acidic volcanic rocks or volcanic ejecta composed predominantly of aluminum silicate with ZrO.sub.2 for enhancing alkali-resistance, CaO for lowering fusing temperature of glass, ZnO especially for preventing devitrification and if necessary Fe.sub.2 O.sub.3 for enhancing devitrification-resistance, lowering of spinning temperature and alkali-resistance. The present invention has been accomplished on the basis of the above finding.